Acidic compounds, such as CO2, H2S and COS can be “scrubbed” with a liquid absorbent medium, referred to herein as a scrubbing agent solvent, and removed from fluid streams under treatment in petrochemical refining processes. Alkaline solutions such as alkanolamines (e.g., monoethanolamine (MEA) or diethanolamine (DEA)), hindered amines, caustic or other appropriate solvents can be employed as scrubbing agents to assist in the removal of the acid gas components.
An operational problem often encountered with such acid gas abatement processes is corrosion of carbon steel and other low-alloy steels that are used in the construction of the piping and vessels. Such corrosion can be attributable to one or more of the following: decomposition of the scrubbing agent solvent, reaction of the acidic components of the gas and the scrubbing agent solvent; and direct attack by the acidic components in the gases.
In addition, acid gas abatement processes can result in the accumulation of heat stable salts (HSS) (for example, due to ingress of reactive contaminants and degradation of amine), which in itself can lead to higher corrosion rate and equipment damage. HSS accumulation also ties up scrubbing agent solvents, reducing the available amount for acid gas absorption. Management of HSS levels can be achieved through a reclamation process or replacement of part of the amine inventory with fresh, uncontaminated amine (bleed and feed or larger bulk replacement). Corrosion inhibition technology is not standard practice in amine treating systems due to the high cost of most programs and undesirable side effects (e.g., foaming). As such, for a typical acid gas treating facility (e.g., an amine scrubbing agent acid gas treating facility) the primary mitigation strategy for corrosion of carbon steel or other low-alloy steels is to replace these materials with more corrosion-resistant stainless steel.
A more recent corrosion control program contemplated for acid-gas treating facilities involves the addition of soluble sodium tetrasulfide to the circulating amine. This program was shown in lab and field studies to form protective iron sulfide layers on carbon steel to reduce corrosion rates. Unfortunately, the high cost of the additive makes this concept economically infeasible. Therefore, the introduction of soluble polysulfide ions was determined to be an effective corrosion mitigation strategy, notwithstanding commercial realities.
Polysulfide ions can be obtained from the air oxidation of sulfide ions that are formed from dissociated hydrogen sulfide in circulating amine solutions. Air oxidation of sulfide ions, however, degrades amine scrubbing agents, produces excessive quantities of additional oxidative HSS by-products that are detrimental to the process, and can react with diolefins to form a polymeric product that fouls equipment. Alternatively, U.S. Pat. Nos. 4,944,917, 4,857,283 and EP 102 712 describe the addition of ammonium or metal polysulfides or other means of forming polysulfide ions into circulating amine treating solutions. Similarly, already-prepared polysulfides could be purchased and added directly to the process stream. Although theoretically successful, the cost of these various chemical addition techniques has proved to be prohibitive in view of their benefits, and has resulted in limited commercial applicability to date.
While the performance benefits of polysulfide ions have been determined, a more cost-effective method of generating the polysulfide ions is desired without the expense of, for example, ammonium or metal polysulfides additives, and without the disadvantages of obtaining polysulfide ions from air oxidized sulfide ions obtained from dissociated hydrogen sulfide. There also remains a need to integrate the polysulfide generation with effective management of corrosion of metal surfaces in chemical or petrochemical operations hydrocarbon refining operations) connection with acid gas removal efforts.